Rhesus monkeys were immunized with bovine myelin basic protein (MBP) and M. tuberculosis H37RA in Freund's Complete Adjuvant. Controls (5/5) developed multiple sclerosis (MS)-like clinical symptoms within 20 days after immunization and were euthanized 3-5 days later. Magnetic resonance imaging and necropsy examination confirmed the presence of MS lesions in the brain and other regions of the central nervous system. Test animals treated orally with varying doses of the synthetic co-polymer Copaxone (COP-1) showed significantly reduced MS-like clinical symptoms. Peripheral blood and cerebral spinal fluid (CSF) lymphocytes were analyzed by flow cytometry to determine shifts in CD4+CD45RA+ T cells, programmed cell death and the presence of activated T cells. Plasma samples were examined by enzyme-linked immunoassay for MBP-specific IgG and IgA antibodies and T suppressor factors. 3H-thymidine incorporation was used as a measure of antigen-induced lymphocyte proliferation. Control and test animals made significant levels of MBP-specific IgG and IgA. CSF lymphocytes of controls showed an increase in the number (>50) of CD4+CD45RA- T cells one week before the onset of clinical symptoms. In contrast, lymphocytes from test animals treated with COP-1 showed a significant increase in CD4+CD45RA+ T cells, suggesting the presence of T suppressor cells. Analysis of plasma collected at various times during these studies indicate that animals treated with COP-1 produced significant levels of MBP-specific T suppressor factors. Plasma from control animals lacked MBP-specific T suppressor factors. Antigen-induced proliferation studies showed that lymphocytes from control and test animals exhibited no proliferation when cultures were pulsed with MBP, but these cells showed strong proliferation when cultured with Con A. These, studies in consort with strong positive skin tests to a purified protein derivative of H37RA (PPD), strongly suggest that the observed suppression of clinical symptoms is not due to the lack of IL-2 (anergy) or apoptosis.